The present invention relates to an (electro) conductive liquid crystal device used in electronic devices and an organic electroluminescence device using the liquid crystal device.
As for organic electroluminescence devices (hereinafter, the term xe2x80x9celectroluminescencexe2x80x9d being sometimes abbreviated as xe2x80x9cELxe2x80x9d according to a common usage in the field), carrier injection-type EL devices utilizing organic solids, such as anthracene single crystal, were studied in detail. These devices were of a single layer-type, but thereafter Tang et al. proposed a lamination-type organic EL device comprising a luminescence layer and a hole transporting layer between a hole injecting electrode and an electron injecting electrode. The luminescence mechanism in these injection-type EL devices commonly includes stages of (1) electron injection from a cathode and hole injection from an anode, (2) movement of electrons and holes within a solid, (3) re-combination of electrons and holes, and (4) luminescence from single term excitons.
A representative example of the lamination-type EL device may have a structure including an ITO film as a cathode formed on a glass substrate, a ca. 50 nm-thick layer formed thereon of TPD (N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-di(3-methylphenyl)-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine) as represented by a structural formula shown below, a ca. 50 nm-thick layer thereon of Alq3 (tris(8-quinolinolato)-aluminum) as also represented by a structural formula shown below, and further a vacuum deposition layer of Alxe2x80x94Li alloy as a cathode. 
By setting the work function of the ITO used as the anode at 4.4-5.0 eV, the hole injection to TPD is made easier, and the cathode is composed of a metal which has as small a work function as possible and also is stable. Examples of the cathode metal may include Alxe2x80x94Li alloy as mentioned above and also Mgxe2x80x94Ag alloy. By the above organization, green luminescence may be obtained by applying a DC voltage of 5-10 volts.
An example using a conductive liquid crystal as a carrier transporting layer is also known. For example, D. Adam et al. (Nature, vol. 371, p. 141) have reported that a long-chain triphenylene compound as a discotic liquid crystal material exhibited a mobility of 10xe2x88x923-10xe2x88x922 cm2/V.sec in its liquid crystal phase (Dh phase) and a mobility of 10xe2x88x921 cm2/V.sec in its mesophase (an intermediate phase, not a liquid crystal phase). Also, as for a bar-shaped liquid crystal, Junichi Hanna (Ohyou Butsuri, Appl. Phys., vol. 68, no. 1, p. 26) has reported that a phenylnaphthalene compound exhibited a mobility of 10xe2x88x923 cm2/V.sec or higher in its smectic B phase.
As a trial for using such a liquid crystal for electroluminescence, Ingah Stapff et al. (Liquid Crystals, vol. 23, no. 4, pp. 613-617) have reported an organic EL device using a triphenylene-type discotic liquid crystal. Other reports are found in POLYMERS FOR ADVANCED TECHNOLOGIES, vol. 9, pp. 463-460 (1998) and ADVANCED MATERIALS, vol. 9, no. 1, p. 48 (1997).
In a conventional organic EL device, a high electric field (on the order of 10 V/100 nm) has been required for drive because of low performances of injection of electrons and holes from the electrodes, such as ITO, to the organic layers. As organic materials used in an organic EL device have a band gap as broad as ca. 3.0 eV or more, thermal excitation-type free electrons are not present in a conduction band (or LUMO: Lowest Unoccupied Molecular Orbital); a drive current is principally supplied by a tunnel current injected from the electrodes. The injection efficiency of the current is known to be remarkably affected not only by the work functions of the electrodes and a level gap between LUMO and HOMO (Highest Occupied Molecular Orbital) of the organic materials, but also by the molecular alignment and structure of the organic materials. For example, in the case where organic molecules assume a crystalline state, minute crystalline boundaries function as carrier conduction obstacles, so that organic materials are generally used in an amorphous state, but only a low carrier injection efficiency is available in this case.
For the above reason, in order to attain a sufficient drive current using ordinary organic compounds (such as TPD, xcex1-NPD (bis[N-(1-naphthyl)-N-phenyl]benzidine), TAZ-01 (3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole), Alq3, etc.) in ordinary EL devices, it has been necessary to apply a high electric field (on the order of 10 V/100 nm) across the organic layer-electrode boundaries. Further, as the mobility of the organic materials is on the order of 10xe2x88x923-10xe2x88x925 cm2/V.sec, it is also necessary to apply a high electric field in order to ensure a drive current.
The application of such a high electric field leads to the necessity of thin device layers, which also lead to an electrical short circuit between the electrodes and an increase in capacitance load.
Further, an organic EL device is liable to be affected by invading moisture which causes deterioration of luminance performance and drive performance, thus causing poor durability. In an ordinary organic EL device, the organic layers are disposed in lamination and then the cathode is formed thereon by vapor deposition of a metal film. In this instance, a metal species having a small work function suitable for the cathode is susceptible to oxidation and has low durability. Even in the case of forming a protective film thereon by sputtering, the organic layers are liable to be degraded if the forming temperature is high (with an ordinary limit of 100xc2x0 C.), and the destruction of the device structure due to film stress is also problematic.
An object of the present invention is to provide a device including a conductive liquid crystal layer functioning as a carrier transporting layer exhibiting a high carrier injection efficiency and good durability.
Another object of the present invention is to provide a reliable organic EL device capable of providing sufficient luminance at a low applied voltage and exhibiting a long-term stable reliability by including such a conductive liquid crystal device.
According to the present invention, there is provided a conductive liquid crystal device comprising a pair of oppositely spaced electrodes and a carrier transporting layer disposed in contact with one of the electrodes and comprising a conductive liquid crystal having a xcfx80-electron resonance structure in its molecule, a protective layer disposed in contact with the carrier transporting layer and having a carrier transporting function, and an organic layer disposed in contact with the protective layer, respectively disposed between the electrodes.
According to the present invention, there is also provided an organic electroluminescence device comprising a pair of oppositely spaced electrodes and a carrier transporting layer disposed in contact with one of the electrodes and comprising a conductive liquid crystal having a xcfx80-electron resonance structure in its molecule, a protective layer disposed in contact with the carrier transporting layer and having a carrier transporting function, and a luminescent organic layer disposed in contact with the protective layer, respectively disposed between the electrodes.
A characteristic device structure according to the present invention is one in which a protective layer having a carrier-transporting function is inserted between an organic layer and a carrier transporting layer; through the protective layer, the deterioration at the boundary between the organic layer and the carrier transporting layer, e.g., the occurrence of molecular association (such as exciplex), can be prevented. Another characteristic is the use of a conductive liquid crystal having a xcfx80-electron resonance structure in addition to the protective layer, by aligning the xcfx80-electron resonance plane of the conductive liquid crystal substantially parallel to the adjacent electrode surface to form a carrier transporting layer, improved performance of carrier injection from the electrode boundary is achieved.
As a result, in the organic EL device according to the present invention, the deterioration at the boundary between the luminescent organic layer and the carrier transporting layer can be suppressed, whereby a sufficient luminescence can be attained at a low voltage thereby reducing power consumption and preventing short circuit between the electrodes and thereby improving the reliability of the organic EL device.
The conductive liquid crystal device of the present invention can be utilized not only in such an organic EL device but also in other electron devices, such as a photosensor, a photoconductor (as in electrophotographic apparatus), an organic semiconductor device (organic TFT), and a spatial modulation device.